Print a histogram of the values present in a PGM image. See also:
ppmhist. More information: https://netpbm.sourceforge.net/doc/pgmhist.html.
pgmhist {{path/to/image.pgm}}
pgmhist {{[-me|-median]}} {{path/to/image.pgm}}
pgmhist {{[-qua|-quartile]}} {{path/to/image.pgm}}
pgmhist {{[-f|-forensic]}} {{path/to/image.pgm}}
pgmhist {{[-ma|-machine]}} {{path/to/image.pgm}}